1. Field of the Invention
The present invention is in the field of resistance weld attachment devices and is directed more particularly to an attachment device for securing insulating material, such as glass fiber batts to the surface of ductwork in order to minimize thermal losses in respect of air passed through the duct. The apparatus may likewise be employed for the fastening of other insulating materials to conductive ferrous substrates.
2. The Prior Art
It is known to minimize thermal losses in ducted installations by employing batts or sheets of insulating material to the interior or exterior of ducts. In typical applications the batts or sheets are glued to internal or external surfaces of the duct material.
In order to assure that the insulating material remains in attached position, it is standard practice to provide secondary securement means, such as, by way of example, an impact fastener having an enlarged head which bears against the surface of the insulation and a specially formed tip which couples to the substrate on impact.
More recently the use of impact fasteners has, by and large, been supplanted by resistance weld pins which likewise have an enlarged head portion and a sharpened tip. The pins are applied by passing the tip through the insulation and into contact with the substrate and causing a welding current to pass through the length of the shank, whereby the tip is melted and fuses with the material of the duct, dependably to anchor the pin in position.
Representative weld pins and methods of attaching the same are shown in U.S. Pat. Nos. 4,429,209 of Jan. 31, 1984 and 3,624,340 of Nov. 30, 1971. Apparatus for automatically welding such pins is shown in U.S. Pat. No. 3,835,285 of Sept. 10, 1974.
While the weld pins and devices of the cited patents have achieved substantial commercial success, in all instances they have required that the duct and batt be disposed atop one of the welding electrodes, normally the ground electrode of the resistance welding apparatus. Attachment is effected by supporting the head of the weld pin in electrical contact with the "live" electrode, the weld circuit, in such instances, running between the ground electrode which engages the duct or substrate, through the substrate shank and head of the welding pin, to the live electrode which engages the head.
As will be apparent from the preceding description, utilization of the resistance weld method has heretofore been possible only where it is readily feasible to engage the exposed surface of the metal substrate with a ground electrode at a position directly opposite the point of application of the weld pin. While such access is easily available when the ducting is being fabricated in a shop, for instance, it is often not feasible in the field to position a ground electrode directly behind the point of application of the weld pin, i.e. in the case of a pre-installed duct.